The objective of this project is to elucidate the structural basis of the binding of small organic blockers to the central cavity of K+ channels, using X-ray crystallographic and electrophysiological techniques. Potassium channels play critical roles in regulating membrane excitability of many diverse cell types. Many K+ channels are blocked by the binding of small molecules within their pores. Some of these blockers are endogenous and play roles in physiological channel function, while others are exogenous molecules-many of great pharmacological importance. We will use electrophysiological and X-ray crystallographic techniques to study the binding mechanisms of two types of small organic blockers: quaternary ammonium (QA) ions and drugs that block the hERG K+ channel. Understanding the K+ channel-blocker interactions at atomic detail will provide critical information for understanding of K+ channel function and regulation. Moreover, it may help to identify and eliminate or re-design drugs that may cause life-threatening side effects such as the long QT syndrome at early stages of drug development. We will use the bacterial K+ channel KcsA as a model system by rationally mutating it to mimic other K+ channels. We will perform electrophysiological studies, and solve the 3-D structures of the model in complex with the blockers.